Use same Docker container for all background commands

[rbren]

Describe the bug The agent frequently gets confused around installing software. It confused the crap out of me too 🙃

It will often run a background command like node server.js. When it sees node not installed, it runs a foreground command of apt install nodejs. But that doesn't install node on the background container!

Steps to Reproduce

1. export SANDBOX_CONTAINER_IMAGE="ubuntu:24.04"
2. Prompt: "write a node server in express"

Expected behavior Node successfully installed and run by agent

Actual behavior Node installed on foreground, but not background